1. Field
The present disclosure relates generally to processing data, in particular, to landing systems. Still more particularly, the present disclosure relates to a method, apparatus, and computer usable program code for detecting data anomalies in a landing system utilizing a global navigation satellite system.
2. Background
An autopilot is a system used to guide a vehicle, such as an airplane, with little or no intervention by a human. Autopilots typically rely on signals transmitted from a ground based station. These signals are used to determine the position of the vehicle with respect to other objects, such as the runway. The autopilot reads its position from a guidance system, such as an instrument landing system (ILS). The autopilot uses error reduction systems to identify and dissipate errors in the navigation information. Errors may occur due to problems such as loss of signal, beam bends, noise, multi-pathing, and oscillatory behavior occurring during over flight interference or fly-through events.
Global navigation satellite system (GNSS) based landing systems (GLS) for aircraft are becoming more widespread as they offer improved accuracy in navigation for takeoff, landing, and autopilots.
GNSS is a navigation system that allows small receivers to determine their position with respect to the earth using signals transmitted from satellites. This system typically permits geo-spatial positioning with world wide coverage. GLS is a system that combines satellite and ground-based navigation information to provide aircraft positional information with respect to a pre-defined approach path during approach, landing and rollout.
A key issue with GLS is the expected failure modes and effects of failures in the GLS guidance system. It is anticipated that the most common failure mode for GLS is the total loss of signal from satellites for hundreds of seconds. Consequently, a GNSS/inertial coasting filter has been developed to provide continuity of service through these outages by continuing to provide inertially derived position information when the GLS guidance is unavailable.
The inertial coasting filter provides aircraft systems with a reliable backup form of guidance when the GLS guidance signals are unusable. The guidance system responds as rapidly as possible to switch to inertial guidance to prevent the inertially based deviations from becoming corrupted by the errors in the GLS signals.
Errors in GLS systems generally occur at a much lower frequency than errors associated with other guidance sources, such as instrument landing systems. However, GLS may experience discrete changes or steps in these steady state errors which occur in fault-free operation of the system as satellites rise and set.
The integrity of a GLS guidance system is frequently specified in terms of an alert limit. An alert limit is a limit on the maximum allowable GLS error on differentially corrected deviations transmitted without annunciation of the problem to the flight crew. However, current standards do not limit the dynamic behavior of the error when the error is within the alert limits. This poses a potential problem for the inertial coasting filter scheme which uses low-frequency information from the nominal GLS deviations to establish an accurate inertial reference.
ILS inertial coasting has a coasting duration limitation of approximately 20 seconds or less which allows adequate time for a backup station to come on line without requiring the approach to be aborted. However, given the potential total loss of GLS signal for hundreds of seconds, to allow an approach to be completed from the alert height in the presence of such failures, the GLS coasting duration must be expanded to 60 seconds or more. To accomplish this safely, more precise inertial coasting is required. Thus, there is a wider range of detrimental error rates for current GLS than ILS systems. For example, if the reference error on the GLS deviations increases at a low enough frequency, the errors will be incorporated into the blended solution via the inertial bias estimate, while the conflicting inertial low-frequency information is rejected. Such a corrupted inertial reference, over time, can lead to touchdown and rollout off the runway. Such problems lead to decreased safety for passengers.
Further complicating the problem is the fact that, due to the high probability of error steps occurring as a result of normal satellite configuration changes, such as during satellite rising and setting, these error events need to be differentiated from error events that require action to be taken to remove negative impact on the GLS auto-land performance. Thus, due to the differences between ILS and GLS, current anomaly detection methods used in ILS based systems cannot be safely and accurately used for anomaly detection in current GLS based navigation systems.
In other words, current GLS landing systems do not employ coast-skip reset filters or take full advantage of the continuity provided by coast-skip reset filters. To enable this, it would be desirable to detect a wide range of ramp errors which are not detectable by current state of the art anomaly detectors.